Apply String.equals() optimizations on arm, arm64 and x86-64.

This is a follow-up to
    https://android-review.googlesource.com/174192

Change-Id: Ie71197df22548d6eb0ca773de6f19fcbb975f065
diff --git a/compiler/optimizing/intrinsics_x86_64.cc b/compiler/optimizing/intrinsics_x86_64.cc
index 06e9cc2..2b626be 100644
--- a/compiler/optimizing/intrinsics_x86_64.cc
+++ b/compiler/optimizing/intrinsics_x86_64.cc
@@ -1416,17 +1416,22 @@
   // Note that the null check must have been done earlier.
   DCHECK(!invoke->CanDoImplicitNullCheckOn(invoke->InputAt(0)));
 
-  // Check if input is null, return false if it is.
-  __ testl(arg, arg);
-  __ j(kEqual, &return_false);
+  StringEqualsOptimizations optimizations(invoke);
+  if (!optimizations.GetArgumentNotNull()) {
+    // Check if input is null, return false if it is.
+    __ testl(arg, arg);
+    __ j(kEqual, &return_false);
+  }
 
-  // Instanceof check for the argument by comparing class fields.
-  // All string objects must have the same type since String cannot be subclassed.
-  // Receiver must be a string object, so its class field is equal to all strings' class fields.
-  // If the argument is a string object, its class field must be equal to receiver's class field.
-  __ movl(rcx, Address(str, class_offset));
-  __ cmpl(rcx, Address(arg, class_offset));
-  __ j(kNotEqual, &return_false);
+  if (!optimizations.GetArgumentIsString()) {
+    // Instanceof check for the argument by comparing class fields.
+    // All string objects must have the same type since String cannot be subclassed.
+    // Receiver must be a string object, so its class field is equal to all strings' class fields.
+    // If the argument is a string object, its class field must be equal to receiver's class field.
+    __ movl(rcx, Address(str, class_offset));
+    __ cmpl(rcx, Address(arg, class_offset));
+    __ j(kNotEqual, &return_false);
+  }
 
   // Reference equality check, return true if same reference.
   __ cmpl(str, arg);